Spectrometer-based techniques, such as spectrophotometry and spectral ellipsometry, are widely used in microelectronics for measuring thin film properties and line profiles. An object to be measured may be a site on a semiconductor wafer (multi-layer stack) that may have with uniform thin-film structure, or may be patterned, e.g., line array in at least one layer of the multi-layer stack. If wavelength of incident light is of the order of the line array period, reflected light is zero-order diffracted light, while higher orders are scattered in different directions and do not reach an optical detector oriented to be capable of detecting the reflected light (scatterometry). For both the specular reflection and zero-order diffraction, the light used for measurements may be polarized. Measurement schemes may be of the kind based on measuring intensity of light (spectrophotometry) or polarization changes (ellipsometry).
An object under measurements, either of uniform or patterned structure may thus be described by an optical model with a set of parameters, such as optical constants, thickness of each layer, pattern geometry and profile. Measurement of a single spectrum is usually incapable of determining more than two-three unknown parameters of this set with high accuracy and high level of confidence. Hence, if more parameters of the optical model of a measured structure are unknown and are to be simultaneously determined from spectral measurements, more such measurements are to be independently carried out. The known approaches to increase the number of independent measurements (applicable for both the spectrophotometry and ellipsometry) are generally based on the following:
1. Step-by-step removal and measurements of the same stack downward to the substrate layer. Although this approach enables as many independent measurements as needed, it is destructive and is hard to implement for measuring wafers in production.
2. Step-by-step deposition and measurements of the stack layers. This approach provides a required number of measurements, but is timely consuming.
3. Individual deposition of each or some critical layers on a substrate and separate measurements of each of these layers. By this, measurements in a simple single-layer structure allows for determining both the optical properties and thickness of the single layer. The single-layer measured parameters can then be excluded from unknown parameters of the whole stack. This technique is also time consuming, requires a large number of test wafers, and does not takes into consideration the fact that the optical properties of a specific layer may be different when the layer is a part of a multi-layer stack, and therefore does not provide confident information.
4. Applying measurements to the same wafer in different ambiences, e.g., in air and in water, etc. This technique provides more information about the wafer, but a different ambience might cause some chemical changes of the wafer's top layer, such as photoresist.
5. Measurements of the same site on a wafer with different angles of light incidence. FIG. 1 schematically illustrates such a variable angle measurement system 10, for example the H-VASE model commercially available from J.A. Woolam Co., Inc. The system has two arms 12 and 14 mounted for pivotal movement about a pivot axis 16 along a curved frame 18. The arms 12 and 14 are L-shaped and carry a light source 20 and a detector 22, respectively. In order to cover the entire surface of a wafer W, the latter is supported on a movable X-Y stage 24, which preferably is also Z-adjustable. This approach provides more information than the single incident angle technique, but suffers from the following drawbacks:                long measurement time, which while being acceptable for material characterization, is not acceptable for production;        a measurement system of a large footprint and size, which does not complies with the requirements for a measurement system in production, e.g., integrated metrology system;        requirement for a massive and rigid mechanical platform, in order to provide accurate changing of the angle of incidence that can hardly be used for integrated metrology.        